A conventionally known gas sensor has a structure in which a sensor element has a detecting portion provided at its front end for detecting a component of an atmosphere and is disposed in the interior of a metal casing. The metal casing is a combined entity of a plurality of tubular members, such as a metallic shell having an externally threaded portion used to mount the sensor; a protector, which is fixed to the metallic shell in such a manner as to cover the detecting portion of the sensor element projecting from the front end of the metallic shell; an outer tube member, which is fixed to a rear-end opening portion of the metallic shell and adapted to protect a portion of the sensor element which extends rearward from the opening portion; and a protective outer tube member, which is disposed in such a manner as to surround the outer tube member from radially outside via a water-repellent filter.
For use, such a gas sensor; for example, an oxygen sensor, is attached to an exhaust pipe or the like of an exhaust system of an automobile engine. According to a recent tendency, a catalytic device is attached to the exhaust pipe for decomposing organic substances contained in exhaust gas, and a gas sensor is attached downstream of the catalytic device for measuring a component of the exhaust gas from which organic substances have been removed. In this case, the gas sensor is disposed at a downstream position of the exhaust pipe, which extends rearward from the engine along a bottom section of the automobile. Accordingly, during travel of the automobile, a splash of water may adhere, in the form of a water droplet, to the outer surface of the gas sensor. Thus, in order to prevent entry of water or the like into the interior of the gas sensor, the plurality of tubular members must be reliably joined together so as to ensure sufficient watertightness of the gas sensor.
An example method for joining the above-mentioned plurality of tubular members together is a crimp-fixing process. For example, in a known gas sensor (refer to, for example, Patent Document 1), the protector is crimp-fixed to a front end portion of the metallic shell; a front end portion of the outer tube member is crimp-fixed to a rear end portion of the metallic shell; and a front end portion of the protective outer tube member which is fitted to the outer tube member is crimp-fixed to the outer tube member. In this gas sensor, for example, a connective crimped portion is provided at a front end portion of the protective outer tube member in such a manner as to be reduced in diameter radially inwardly. A front end portion of the connective crimped portion is in close contact with the outer surface of the outer tube member, thereby preventing penetration of water or the like along the interface between the outer tube member and the protective outer tube member.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H11-352095